A typical passenger-seat airbag device set in a vehicle includes, for example, a folded airbag, a retainer to which the airbag is attached, an inflator (gas generator) for inflating the airbag, and an interior panel (such as an instrument panel or a lid) disposed in front of the retainer.
When the inflator generates gas, the airbag starts to inflate and pushes open the interior panel. The interior panel ruptures along a tear line to allow the airbag to deploy to the vehicle interior, whereby the airbag restrains the occupant.
Japanese Unexamined Patent Application Publication No. 2003-137054 discloses a door-portion-lift-type airbag device that is configured to lift an airbag-passing door portion of an instrument panel towards the vehicle interior at the time of inflation of an airbag so as to allow the door portion to be opened smoothly.
FIG. 9 is a cross-sectional view of the airbag device disclosed in Japanese Unexamined Patent Application Publication No. 2003-137054. FIG. 10 is a cross-sectional view of the airbag device in operation. FIG. 11 is a perspective view of a door frame of the airbag device.
Referring to FIG. 9, an airbag 52 (FIG. 10) in a folded state is housed in a retainer 50 of an upper-opening container type and is inflatable by means of an inflator 54.
The upper side of the retainer 50 is covered with an instrument panel 60. The instrument panel 60 is provided with recessed tear lines 62a and 62b. The tear line 62b extends along the upper rim of the retainer 50, and the inner region surrounded by this peripheral tear line 62b defines a door portion 64. On the other hand, the tear line 62a extends so as to traverse the middle of the door portion 64.
Under the instrument panel 60 is provided a door frame 70 constituted by a movable door-frame member 72 and a stationary door-frame member 74. The stationary door-frame member 74 is rectangular and has the movable door-frame member 72 fitted therein in a slidable fashion.
The movable door-frame member 72 has a backing plate 72a that underlies the door portion 64 of the instrument panel 60, and leg plates 72b extending downward in FIG. 9 from opposite edges of the backing plate 72a. The leg plates 72b are provided with hook holes 72c. The backing plate 72a has a tear line 72d in alignment with the tear line 62a. Along the edges where the backing plate 72a and the leg plates 72b meet are provided recessed hinge grooves 72e. 
The backing plate 72a is fixed to the undersurface of the door portion 64 of the instrument panel 60 by vibration welding.
The stationary door-frame member 74 includes a rectangular leg frame 74a serving as a leg portion that surrounds the outer periphery of the retainer 50, and a flange portion 74b that extends outward from the upper end of the leg frame 74a. The flange portion 74b is fixed to an area of the instrument panel 60 surrounding the door portion 64 by vibration welding. The leg frame 74a is provided with hook holes 74c. 
The retainer 50 has uncinated hooks 78 fixed thereto, which are inserted through the hook holes 72c and 74c. 
Referring to FIG. 10, when the inflator 54 discharges gas, the movable door-frame member 72 is moved (lifted) upward by being pushed upward by the inflating airbag 52. The instrument panel 60 ruptures along the tear line 62b, causing the door portion 64 to be cut and separated from the surrounding instrument panel 60. As the door portion 64 is moved further upward together with the movable door-frame member 72 and is pushed upward by the airbag 52, the tear line 72d of the backing plate 72a and the tear line 62a of the door portion 64 become ruptured, thus causing the door portion 64 to open in the form of a door by being separated into two segments in the left and right directions in FIG. 10. As a result, the airbag 52 deploys toward the vehicle interior.
Because the door portion 64 lifted in this manner becomes separated from the surrounding instrument panel 60, the door portion 64 can be opened smoothly without receiving a deformation reaction force from the surrounding instrument panel 60.
In the airbag device shown in FIGS. 9 to 11, when the airbag 52 inflates and lifts the movable door-frame member 72 upward, a large force that tries to detach the door portion 64 of the instrument panel 60 from the backing plate 72a acts on the joint sections between the backing plate 72a and the leg plates 72b. 
This phenomenon will be described with reference to FIG. 12. FIG. 12 is a schematic cross-sectional view showing a state immediately after the start of the lifting operation.
Referring to FIG. 12, when the movable door-frame member 72 and the door portion 64 are being lifted, the airbag 52 pushes the door portion 64 so that an area near the central tear line 72d forms an upward projecting bulge. In this state, the door portion 64 of the instrument panel 60 is cut and separated from its surroundings and has a rectangular shape with a size substantially the same as that of the backing plate 72a. Since this rectangular door portion 64 tries to maintain its flatness with its own rigidity as shown with a dot-chain line, opposite edges 64S of the door portion 64 try to detach themselves from the backing plate 72a. This detachment tends to occur at corner sections of the backing plate 72a proximate to the leg plates 72b. 
In order to prevent such detachment of the opposite edges 64S of the door portion 64, it is necessary to carefully attach the instrument panel 60 and the backing plate 72a to each other by vibration welding. For this reason, the vibration welding process and the management thereof are time consuming.